The present invention relates to a method of micromechanical manufacturing of fixed and movable layer-like electrodes of a semiconductor element, for example, a capacitive acceleration sensor, which are exposed over a substrate over a certain area, a sacrificial layer arranged between the substrate and the fixed and movable electrodes being removed in an etching step in order to expose the electrodes with respect to the substrate, and an acceleration sensor thus manufactured.
It is believed that in manufacturing semiconductor elements using micromechanical technology, such as acceleration and rotational speed sensors, for example, a silicon dioxide sacrificial layer may be selectively etched in order to obtain the functional components made of silicon. This reactive etching of the sacrificial layer may take place in the gas phase via an azeotropic mixture of H2O and HF. See, for example, M. Offenberg et al., xe2x80x9cAcceleration Sensor in Surface Micromachining for Airbag Applications with High Signal/Noise Ratioxe2x80x9d; Sensors and Actuators, 1996, page 35, and German Published Patent Application No. 4 317 274.
In the event of insufficient progress during the process, oxide residues may remain underneath the functional component layer, which may result in interference and inaccurate response behavior in sensors, for example, acceleration sensors having non-movable electrode fingers fixed on one side. These residues may appear at technologically unfavorable points which are characterized by the fact that, after the sacrificial layer has been etched, the functional layer over them is arched upward from the etching front due to the intrinsic stress gradient, and in the process end state the distance between the bottom of the structure layer and the top of the sacrificial layer becomes greater. There are technologically favorable points where the distance between the surface of the sacrificial layer and the bottom of the structure layer underneath the components anchored on both sides, such as elastically suspended seismic masses of an acceleration sensor, may be reduced due to the intrinsic stress gradient. An increased etching rate may result, which may cause considerable inhomogeneity of the etching rate within a sensor structure, which may result in oxide residues underneath the fixed electrodes and leaves considerable underetching in other places.
The considerably inhomogeneous etching rate underneath the non-movable electrode fingers fixed on one side and the movable central mass of an acceleration sensor may result in a criterion for ending the etching process that is difficult to define when such acceleration sensors are mass produced. Considerable unwanted underetching may occur at several points of the component, for example, at the electric leads, while substantial oxide residues affecting the functionality of the sensor structure still exist in the area of the sensor core underneath the fixed electrodes.
An exemplary method of the present invention is directed to providing a method which allows micromechanical manufacture of components in view of the difficult of the microscopically inhomogeneous etching rate in the manufacturing of semiconductor elements, where layer-like electrode areas are exposed using sacrificial layer etching, as in the case of fixed, comb-like electrodes of an acceleration sensor.
Another exemplary method of the present invention is directed to providing a method which allows the micromechanical manufacture of components in which the etching rate in selective isotropic etching of silicon dioxide in an H2O/HF gas phase can be homogenized on a microscopic scale.
Another exemplary embodiment and/or exemplary method of the present invention is directed to reducing the thickness of the sacrificial oxide layer underneath the non-movable electrode fingers fixed on one side. Thus the etching rate may be substantially increased for kinetic reasons at these points and may approach the etching rate underneath the seismic mass. It is believed that reduction of the thickness of the sacrificial oxide layer underneath the fixed electrode fingers may be achieved by correspondingly increasing the thickness of the epitaxial polycrystalline silicon layer formed over the sacrificial layers which is the material of the subsequently etched bare electrode fingers.
In another exemplary embodiment and/or exemplary method of the present invention, the total time of the gas phase etching process (GPA) may be substantially reduced by adjusting the etching rate underneath the seismic mass and the fixed electrode fingers. This may also provide improved homogeneity of the oxide removal from the sacrificial layer or sacrificial layers within a semiconductor element, for example, an acceleration sensor structure, over the entire wafer.
Another exemplary method of the present invention is directed to providing a method of micromechanical manufacture of fixed and movable layer-like electrodes of a semiconductor element, for example, a capacitive acceleration sensor, which may be exposed over a substrate over a certain area, a sacrificial layer arranged between the substrate and the fixed and movable electrodes being removed in an etching step in order to expose the electrodes with respect to the substrate. The thickness of the sacrificial layer located in the area of the fixed electrodes may be less than the thickness of the sacrificial layer located in the area of the movable electrodes.
Another exemplary method of the present invention is directed to providing a method where the thickness of the fixed electrodes after sacrificial layer etching is greater than the thickness of the movable electrodes. The increase in the thickness of the fixed electrode fingers of a sensor structure of this kind may result in the following effects.
For example, if the functional structures had the same layer thickness, the effective capacitive electrode area may be reduced, since a fixed electrode finger may be bent upward due to the stress gradient. Since at the same time the seismic mass, fixed on both sides, may arch downward, the electrode area of the opposite electrodes may be effectively reduced.
For example, if the fixed electrodes have a greater thickness than the movable electrodes, the central mass itself may still be opposite the counterelectrode in the event of a high stress gradient.
In the proposed layer structure, the sacrificial layer underneath an area of the fixed electrodes may be composed of a first sacrificial layer, and the sacrificial layer underneath an area of the movable electrodes may be composed of the above-named first sacrificial layer and a second sacrificial layer located directly over it.
Another exemplary method of the present invention is directed to providing a method of manufacturing in the following consecutive steps:
a) applying the first sacrificial layer to the entire surface of the substrate so that it covers the areas where the fixed and movable electrodes are subsequently formed;
b) applying a first conductive layer over the first sacrificial layer so that it only covers the area where the fixed electrodes are subsequently formed;
c) applying the second sacrificial layer to the entire surface of the first sacrificial layer and the first conductive layer so that it covers the areas where the fixed and movable electrodes are subsequently formed;
d) opening the second sacrificial layer via the first conductive layer using a masked etching step so that the first conductive layer is exposed and only a small portion of the depth of the first sacrificial layer is etched away;
e) applying a relatively thick doped epitaxial layer to the entire surface of the second sacrificial layer and the conductive layer exposed in step d) to the thickness of the fixed and movable electrodes to be subsequently formed;
f) applying a mask structuring the fixed and movable electrodes to the surface of the epitaxial layer, and, using the mask, ditches are etched into the epitaxial layer to a depth delimited by the surface of the first sacrificial layer in the area of the fixed electrodes and by the surface of the second sacrificial layer in the area of the movable electrodes; and
g) isotropically etching the first and second sacrificial layers, resulting in approximately the same underetching rates of the sacrificial layer underneath the fixed electrodes and of the sacrificial layers underneath the movable electrodes.
Another exemplary embodiment and/or exemplary method of the present invention is directed to providing that the thickness of the first sacrificial layer is in the range of 0.5-5 xcexcm and the thickness of the second sacrificial layer is in the range of 0.5-3 xcexcm.
Another exemplary embodiment and/or exemplary method of the present invention is directed to providing that the thickness of the fixed electrodes is 1.5-20 xcexcm and the width of the fixed and movable electrode strips or electrode fingers of an acceleration sensor manufactured according to the exemplary embodiment and/or exemplary method of the present invention is in the range of 1-5 xcexcm.
Another exemplary method of the present invention is directed to providing a method where both the thickness of the seismic mass and that of the fixed electrode finger in the area of the sensor core may be increased to the same extent. In doing so, it should be ensured that the thickness of the sacrificial layer is not reduced in the area of the suspension points. The function layer may be connected to the conductor via a bridge underneath the fixed electrode. This mechanical connection may not be destabilized during the gas phase etching because the sacrificial layer is thicker underneath the movable electrode structures than in the area underneath the fixed electrode structures.
Another exemplary embodiment of the present invention is directed to providing an acceleration sensor manufactured using the exemplary method of the present invention which has a plurality of movable electrodes projecting from an elastically suspended movable mass designed as a central bar, alternating with non-movable electrodes fixed on one side or on both sides, which are opposite the movable electrodes. The electrodes fixed on one side may have a conductive electrode strip connecting them on their end facing away from the central bar of the movable mass and directed perpendicular to the teeth of the comb-like electrodes.
Another exemplary embodiment and/or exemplary method of the present invention is directed to providing that the etching rate is the same underneath the fixed electrodes and the movable central mass, so that the acceleration sensors or rotational speed sensors may be mass manufactured with a considerably higher yield and higher reliability.